Internal, self-closing stop valves (“internal valves”) have often been used as a main valve on small capacity pumping systems or in vapor return lines on trucks to provide vapor equalization. While typically designed for propane, butane, or NH3 tanks at ambient temperatures, the valves can be used with other compressed liquids and/or gases. Department of Transportation regulations generally require the use of an internal valve for each liquid or vapor discharge outlet on cargo tanks. For example, Department of Transportation regulations require certain cargo tanks transporting propane, anhydrous ammonia, and other liquid compressed gases to be equipped with passive emergency discharge control equipment that will automatically shut off the flow of product without human intervention within 20 seconds of an unintentional release caused by separation of a delivery hose.
A typical internal valve contains an excess flow function, or integrated excess flow valve, that will close when the flow exceeds an established flow rating. An internal valve installed on a cargo tank typically provides protection against the discharge of hazardous materials during an unloading operation in the event that a pump and/or piping attached to the internal valve is sheared off and/or otherwise breached. Similarly, an internal valve installed on a stationary tank will provide protection against the discharge of hazardous materials in the event that a pump and/or piping attached to the internal valve is sheared off and/or otherwise breached.
An internal valve often requires the use of a remote operating control system, such as for example, a cable control, and/or a powered actuator system. Typically, the operating linkage employed allows the operating lever to move between a fully closed position and a fully open position. In one example, a pneumatic actuator, such as a spring return brake actuator, may be used to remotely operate the internal valve. While these actuators provide for fast operation of the internal valves between the fully closed position and the fully open position, the uncontrolled movement allows less time for the valve to be in a mid-point or rapid bleed position and, thus, result in a longer time necessary to equalize pressure and allow the valve to open.